For decades, if not centuries, mankind has been seeking mechanisms and methods for removing particulates from a flowing stream of gaseous material. The number and type of such filtration devices has generally only been limited by the imagination and creativity of those in the field. Filters, having gained wide acceptance in an extraordinary variety of fields, are under constant evaluation for potential improvement in either their filtration efficiency or their cost or both. Their utility in both industrial, commercial and household applications is beyond question.
A variety of different filtration mechanisms have evolved over the years. For example, mechanical entrapment of particulates entrained within a stream of, for example, air which is being passed through the filter is one well known mechanism. This mechanism, however, is not without its limitations. As the need and desire to remove smaller and smaller particulates with an even increasing higher and higher desired percentage of particulate removal (% efficiency) has grown, it has become apparent that certain limitations exist with mechanical entrapment type filters. Generally speaking, for mechanical entrapment to occur, the pores of the filtration media must be smaller than the particulates which are to be entrapped. Otherwise, the particulates will merely pass through the filter resulting in an undesirable filtration efficiency. Unfortunately, as the pore size of the filtration media is reduced, the ability of the gaseous fluid (air) to pass through the filter is, likewise, reduced. An undesirable side effect is a concomitant reduction in the amount of gaseous material which can pass through the filtration media in a given period of time results. Further, this situation also results in the creation of a significant pressure drop between the two sides of the filter placing the entire filter structure under mechanical duress. In response, stronger filtration media materials have evolved and been utilized. However, removal of very small particulates remained a challenge.
In response to the limitations placed upon the abilities of filters to remove very small particulates, those of skill in the art turned to other mechanisms of particulate removal. One highly satisfactory method was to form the filtration media from a dielectric material. That is, a material which can retain a charge for an extended period of time. The dielectric material of the filter was then subjected to charging as, for example by conventional electreting processes. Exemplary of these processes is a method which applies a charge as a result of the material being subjected to a DC corona discharge treatment. Because the filtration media maintains a charge, it will attract very fine particles having an opposite electrical charge. Further, because the mass of these very fine particulates is so small, the attractive charge is sufficient to retain, that is filter out, the very fine particulates from a stream of air or other gas passing through the filter. Charging a dielectric filter material allowed those of skill in the art to contemplate a range of new possibilities. For example, the pores of the filtration media could be maintained at the smallest possible sized for mechanical entrapment and even smaller particulates would be removed as a result of the charge. Alternatively, the pores of the filtration media could be enlarged to reduced the pressure drop between the two sides of the filter. In such situations filtration efficiencies comparable to filters having smaller pores could be achieved as a result of the additional filtration efficiency of the charged filter material.
Filters made from a dielectric material which had been charged function very well and in most instances, depending upon design, superior to filters which rely solely upon mechanical entrapment. Yet, as is well known, filters relying solely upon mechanical entrapment still flourish. One of the main reasons that filters made from charged dielectric materials have not removed mere mechanical entrapment filters from the marketplace is cost. This cost differential is largely tied up in the cost of the dielectric material as compared to the cost of non-dielectric filtration media.
Accordingly, a need exists in the art for a filtration media which provides both mechanical entrapment and charge entrapment and which is more economical to manufacture than filters made from charged dielectric materials.